Hidden Machine

Hidden Machines (abbreviated as HMs) are items shaped like VCDs or DVDs. In addition to what Technical Machines do -- teaching Pokémon certain attacks, they also grant important field usages such that players cannot complete the game without them. They have been reusable from the very first day they were introduced, and these attacks cannot be forgotten by normal means.

Instruction
The procedure of using a HM is identical to how a TM is deployed -- simply open the bag, click on "TMs/HMs", choose a HM and press "Use". Click on a compatible Pokémon (which will be listed as "Able") to teach that Pokémon the move. Pokémon which cannot learn the attack are listed as "Unable" in red, and Pokémon which have already learnt the attack are listed as "Learnt" in blue.

The remaining process is similar to how a Pokémon learns attacks upon levelling up -- the Pokémon will have to delete a move to make room if it already possesses 4 moves. However, once learnt, Pokémon cannot forget the move unless taken to the Move Deleter in Battle Colosseum.

Each HM also provides an unique field usage. Some are accessed by clicking the required object, such as clicking on a small tree to use Cut, while Fly is used by accessing the Pokémon screen in menu. All HMs can be used as in-battle attacks without any restriction, but field usages are limited such that certain gym badges are required to unlock.

List of HMs
Here is a list of currently available HMs. Field usages are in italics.